1. Field of the Invention
The present invention relates to a transmission control circuit in a mobile radio communication apparatus. Conventionally, speech signals are transmitted through a narrow-band single radio channel. Recently, digital data is also transmitted through the narrow-band single radio channel. Further, the communication is often carried out between a mobile radio communication apparatus and a mobile or immobile radio communication station.
2. Description of the Related Art
For example, the mobile radio communication apparatus is often carried or mounted on a moving vehicle such as a car, and the communication is carried out between the mobile radio communication apparatus in the moving vehicle and one of multiple immobile base stations provided at a predetermined plurality of places for mobile radio communication. When transmitting or receiving radio signals in a mobile radio communication apparatus, a first intensity of radio waves transmitted from the mobile radio communication apparatus toward the base station, and a second intensity of radio waves transmitted from the base station to the mobile radio communication apparatus vary depending upon the relative positions of the mobile radio communication apparatus, the base station, and obstacles temporarily located between the mobile radio communication apparatus and the base station. FIG. 1 illustrates a typical situation in mobile radio communication, where the mobile radio communication is carried out between the mobile radio communication apparatus in a car 1 and a base station 2, and the obstacles are buildings 3.
FIG. 2 indicates an example variation of a strength of an electric field detected by the mobile radio communication apparatus in the moving car 1. When the strength of an electric field detected by the mobile radio communication apparatus varies as indicated in FIG. 2, the data transmitted with the electric field is lost while the strength is below a predetermined level (for example, 20 dB) as indicated with "b" in FIG. 2, and the data transmitted with the electric field may be changed while the strength rapidly falls as indicated by "a" in FIG. 2. When the intensity of radio waves transmitted from the base station to the mobile radio communication apparatus varies, the intensity of radio waves transmitted from the mobile radio communication apparatus toward the base station is considered to similarly vary. Therefore, it is advantageous to stop the transmission of data from the mobile radio communication apparatus to the base station when the intensity of radio waves transmitted from the base station to the mobile radio communication apparatus is below a predetermined level, or when the intensity rapidly falls, to prevent a serious error in data transmitted with the radio waves transmitted from the base station to the mobile radio communication apparatus.